According to the GSM standard, for example, a GSM mobile station (mobile phone) uses the SIM card of a subscriber, which contains a secret identifying the SIM card, the secret being also known to the network operator (shared secret), and a PIN number requested from the subscriber using the mobile radio station. A network operator can use an appropriate protocol (e.g. the challenge response protocol for GSM authentication) to identify a user's SIM card and to permit or refuse the subscriber use of the mobile radio network, for example. This method is, however, only suitable for authentication in n:1 relationships (authentication of, for example, n potential subscribers of a mobile radio network with respect to one network operator) but is not suitable for authenticating the user with respect to a plurality of potential business partners (not known definitively in advance) (n:m relationship).